How long has this been going on?
What other health issues do you have, any heart problems?
There is really too many causes and without more info I can not say what your experiencing is.
I am sorry you are having this pain. Sandee is correct there are numerous thing this pain could be. Because you are so young it is not more apt to be a muscle or possible GERD, (gastrointestinal reflux disease). However one should never R/O (rule out) cardiac issues.
I would consult with my doc to see what could be causing this. I hope you are soon feeling better. Keep us posted and take care, Tuck